Internal combustion engineers have long recognized the desirability of reducing the weight of the various components in an internal combustion engine. Concommitantly, engineers have sought to simplify the casting and molding of these various components in order to reduce the costs of producing the engine.
It is often difficult to meet simultaneously the goals of reducing both engine weight and complexity, but relatively recent efforts by the internal combustion engine industry have resulted in new engine design concepts which incorporate several heretofore conventional features of the engine into one lightweight, simplified structural unit. For example, U.S. Pat. No. 3,411,490, issued to Akana on Nov. 19, 1968, and U.S. Pat. No. 3,832,983, issued to Nickly on Sept. 3, 1974, both disclose cylinder heads for internal combustion engines wherein longitudinal recesses formed in the sides of the cylinder heads are used in lieu of conventional intake manifolds to provide a receiving chamber or plenum from which intake air is distributed to the cylinders. Akana in particular recognizes the advantages of reduced weight and lessened fabrication costs inherent in a combined intake manifold/cylinder head arrangement. Despite the utility of both the Akana and Nickly inventions, however, the weight and cost advantages obtained by combining the intake manifold with the cylinder head are limited by the fact that the cylinder head itself is generally cast from a sand mold which must be rebuilt after each casting. Consequently, even though a combined intake manifold/cylinder head assembly may be less complex to reproduce than separate intake manifold and cylinder head components, truly significant reductions in the casting costs of the cylinder head can only be achieved if the air intake collection and distribution structure is at least partially removed from the head and placed in an engine component capable of manufacture by a reusable die.
U.S. Pat. No. 3,973,548, issued to Celli on Aug. 10, 1976, discloses an internal combustion engine with die cast static parts including a die cast cylinder head attached to the engine cylinder casting and a die cast valve train casing secured to the top of the cylinder head. A group of air passageways formed in the interior of the valve train casing communicate with respective intake passageways in the cylinder head to supply air at atmospheric pressure to the intake ports of the cylinders. Such placement of a portion of the air intake system in the valve train casing results in a simplified cylinder head construction, thus enabling Celli to employ cost and weight efficient die-casting techniques in the manufacture of his engine components. Nevertheless, the teachings of Celli are not readily adaptable to all types of internal combustion engines. In particular, those engines which require pressurized or otherwise pre-conditioned intake air will be unable to benefit from the Celli valve train casing. This is because the air passageways formed in the valve train casing all open directly to the atmosphere, and Celli makes no provision for collecting his intake air in a manifold-type structure prior to distributing the air to the cylinders. In situations where, for example, turbocharging or aftercooling capability is required, it is necessary to construct an engine component separate from the cylinder head yet able to centrally receive and collect the intake air from the pre-conditioning source prior to distribution if the weight and cost advantages discussed in Celli are to be fully exploited.
Apart from internal combustion air intake systems, engineers have turned to other portions of the engine in their search for ways to reduce engine weight. One solution to the weight problem has been to employ plastic engine components wherever possible. U.S. Pat. No. 4,101,003, issued to Timour et al. on July 18, 1978 and assigned to the assignee of the present invention, discloses an oil pan assembly which may be molded from plastic. Other engine components, if properly designed, could also be formed from plastic material. However, as pointed out by Timour et al, plastic is particularly susceptible to engine vibration and plastic engine components often produce undesirable levels of noise. Consequently, the proper design of such components requires the provision of means to damp vibrations arising during operation of the engine.
The isolated rocker arm cover disclosed in U.S. Pat. No. 4,027,644, issued to Timour on June 7, 1977 and also assigned to the assignee of the present invention, directly addresses the problem of damping noise-producing vibrations in internal combustion engine components. Timour utilizes an elongated elastomeric seal element which extends from a groove formed around the periphery of the rocker arm cover to isolate the cover from the engine, thereby damping vibrations which would otherwise be transmitted directly from the engine to the cover. Bolts with resilient fastener assemblies hold the cover in place, and elastomeric grommets surround the bolts to prevent any indirect transfer of vibrations between the cover and the engine. Although Timour provides an adequate means for diminishing the noise emanating from the rocker arm cover, the width of the elongated elastomeric element which seals the periphery of the rocker arm cover is not sufficient to prevent oil leakage therethrough during periods of heavy engine vibration. The provision of a more substantial sealing arrangement around the rocker arm cover is thus necessary if the vibration-damping features of Timour are to be effectively applied to heavy-duty internal combustion engines subject to large vibrational stresses.